Forming doped regions in semiconductor strips

ABSTRACT

A method includes etching a semiconductor substrate to form trenches, with a portion of the semiconductor substrate between the trenches being a semiconductor strip, and depositing a dielectric dose film on sidewalls of the semiconductor strip. The dielectric dose film is doped with a dopant of n-type or p-type. The remaining portions of the trenches are filled with a dielectric material. A planarization is performed on the dielectric material. Remaining portions of the dielectric dose film and the dielectric material form Shallow Trench Isolation (STI) regions. A thermal treatment is performed to diffuse the dopant in the dielectric dose film into the semiconductor strip.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.15/281,568, entitled “Forming Doped Regions in Semiconductor Strips,”filed on Sep. 30, 2016, which application is incorporated herein byreference.

BACKGROUND

With the increasing down-scaling of integrated circuits and theincreasingly demanding requirements to the speed of integrated circuits,transistors need to have higher drive currents with increasingly smallerdimensions. Fin Field-Effect Transistors (FinFET) were thus developed.The FinFETs include vertical semiconductor fins above a substrate. Thesemiconductor fins are used to form source and drain regions, and toform channel regions between the source and drain regions. ShallowTrench Isolation (STI) regions are formed to define the semiconductorfins. The FinFETs also include gate stacks, which are formed on thesidewalls and the top surfaces of the semiconductor fins.

There are various regions that may be formed by implantations. Forexample, anti-punch-through stoppers may be formed by performing tiltedimplantations after the etching of a semiconductor substrate to formtrenches, which trenches are filled in subsequent steps to form STIregions. The remaining semiconductor strips defined by the trenches havehigh aspect ratios. Accordingly, the implanted species are unable toreach the bottom portions of the semiconductor strips since theimplanted species are blocked by neighboring semiconductor strips.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 8A and FIG. 8B are cross-sectional views of intermediatestages in the formation of Shallow Trench Isolation (STI) regions andFin Field-Effect Transistors (FinFETs) in accordance with someembodiments.

FIG. 9 illustrates a cross-sectional view of an n-type FinFET and ap-type FinFET in accordance with some embodiments.

FIG. 10 illustrates a process flow for forming STI regions and a FinFETin accordance with some embodiments.

FIG. 11 illustrates a schematic distribution profile of a dopantinitially doped in a dose film in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Shallow Trench Isolation (STI) regions, Fin Field-Effect Transistors(FinFETs), and the methods of forming the same are provided. Theintermediate stages in the formation of the STI regions and the FinFETsare illustrated in accordance with exemplary embodiments. Somevariations of some embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIGS. 1 through 8A illustrate the cross-sectional views of intermediatestages in the formation of STI regions and a FinFET in accordance withsome embodiments. The steps shown in FIGS. 1 through 8A are alsoillustrated schematically in the process flow shown in FIG. 10.

Referring to FIG. 1, semiconductor substrate 20, which is a part ofsemiconductor wafer 100, is provided. In accordance with someembodiments of the present disclosure, semiconductor substrate 20includes crystalline silicon. Other commonly used materials, such ascarbon, germanium, gallium, boron, arsenic, nitrogen, indium, and/orphosphorus, and the like, may also be included in semiconductorsubstrate 20. Semiconductor substrate 20 may also be formed of othersemiconductor materials such as III-V compound semiconductor materials.Semiconductor substrate 20 may be a bulk substrate or aSemiconductor-On-Insulator (SOI) substrate.

Pad layer 22 and mask layer 24 are formed on semiconductor substrate 20.Pad layer 22 may be a thin film including silicon oxide. In accordancewith some embodiments of the present disclosure, pad oxide layer 22 isformed in a thermal oxidation process, wherein a top surface layer ofsemiconductor substrate 20 is oxidized. Pad layer 22 acts as an adhesionlayer between semiconductor substrate 20 and mask layer 24. Pad layer 22may also act as an etch stop layer for etching mask layer 24. Inaccordance with some embodiments of the present disclosure, mask layer24 is formed of silicon nitride, for example, using Low-PressureChemical Vapor Deposition (LPCVD). In accordance with other embodimentsof the present disclosure, mask layer 24 is formed by thermalnitridation of silicon, Plasma Enhanced Chemical Vapor Deposition(PECVD), or plasma anodic nitridation. Mask layer 24 is used as a hardmask during subsequent photolithography processes. Mask layer 26 isformed over mask layer 24 and is then patterned, forming openings 28 inmask layer 26. Mask layer 26 may be formed of photo resist or a hardmask material such as silicon nitride.

Referring to FIG. 2, mask layer 24 and pad layer 22 are etched, exposingunderlying semiconductor substrate 20. The exposed semiconductorsubstrate 20 is then etched, forming trenches 32. The respective step isshown as step 202 in the process flow shown in FIG. 10. The portions ofsemiconductor substrate 20 between neighboring trenches 32 are referredto as semiconductor strips 30 hereinafter. Trenches 32 may have theshape of strips (when viewed in the top view of wafer 100) that areparallel to each other, and trenches 32 are closely located from eachother. In accordance with some embodiments of the present disclosure,the aspect ratio (the ratio of depth to width) of trenches 32 is greaterthan abut 7, and may be greater than about 10.

After the etching of semiconductor substrate 20, mask layer 26 (FIG. 1)is removed. Next, a cleaning step may be performed. The cleaning may beperformed using diluted hydrofluoric (HF) acid, for example.

In accordance with some embodiments, liner oxide 34 is formed at thebottoms of trenches 32 and extending on the sidewalls of semiconductorstrips 30, as is shown in FIG. 3. The respective step is shown as step204 in the process flow shown in FIG. 10. Liner oxide 34 may be aconformal layer, whose horizontal portions and vertical portions havethicknesses close to each other. The thickness of liner oxide 34 may bein the range between about 10 Å and about 100 Å. In accordance with someembodiments of the present disclosure, liner oxide 34 is formed byoxidizing wafer 100 in an oxygen-containing environment, for example,through Local Oxidation of Silicon (LOCOS), wherein oxygen (O₂) may beincluded in the respective process gas. In accordance with otherembodiments of the present disclosure, liner oxide 34 is formed usingIn-Situ Steam Generation (ISSG), for example, with water steam or acombined gas of hydrogen (H₂) and oxygen (O₂) used to oxidizesemiconductor strips 30. The ISSG oxidation may be performed at anelevated temperature. In accordance with yet other embodiments, lineroxide 34 is formed using a deposition technique such as Sub AtmosphericChemical Vapor Deposition (SACVD). The formation of liner oxide 34 mayresult in the rounding of the corners of trenches 32, which reduces theelectrical fields of the resulting FinFET, and hence improves theperformance of the resulting integrated circuit. In accordance withalternative embodiments of the present disclosure, the formation ofliner oxide 34 is skipped. Accordingly, the subsequently formed dosefilm 36 (FIG. 4) is in physical contact with substrate 20.

During the formation of liner oxide 34, liner oxide 34 is not doped withadditional n-type dopant (such as phosphorous, arsenic, antimony) andp-type dopant (such as boron and indium). Accordingly, the p-type andn-type dopant concentration in liner oxide 34 may be equal to or closeto zero when it is formed by deposition, or may be close to therespective dopant concentration in substrate 20 and semiconductor strips30 when it is formed by oxidizing substrate 20 and semiconductor strips30.

FIG. 4 illustrates the deposition/formation of dose film 36, which isalso a dielectric layer. The respective step is shown as step 206 in theprocess flow shown in FIG. 10. In the embodiments wherein liner oxide 34is formed, liner oxide 34 is in contact with semiconductor strips 30,and dose film 36 is formed on, and contacting, liner oxide 34. Inaccordance with alternative embodiments of the present disclosure, lineroxide 34 is not formed, and hence dose film 36 is in contact with thesidewalls of semiconductor strips 30 and the top surfaces of theunderlying substrate 20. In accordance with some embodiments of thepresent disclosure, dose film 36 includes a base material and a p-typeor an n-type dopant, and the conductivity type of the dopant depends onthe regions to be formed by dose film 36, as is discussed in detail insubsequent paragraphs. During the deposition of dose film 36, the p-typeor the n-type dopant is in-situ doped with the dopant with theproceeding of the deposition. In accordance with some embodiments of thepresent disclosure, dose film 36 includes an n-type dopant such asphosphorous, arsenic, antimony, or combinations thereof. In accordancealternative embodiments of the present disclosure, dose film 36 includesa p-type dopant such as boron, indium, or combinations thereof.

The base material of dose film 36 includes silicon oxide (SiO₂) inaccordance with some embodiments. In accordance with alternativeembodiments, the base material of dose film 36 includes silicon nitrideor other dielectric materials such as silicon carbide, siliconoxynitride, silicon oxynitride, or the like. Dose film 36 may be formedusing Chemical Vapor Deposition (CVD), Atomic Deposition (ALD), LowPressure Chemical Vapor Deposition (LPCVD), or the like. In accordancewith some embodiments of the present disclosure, dose film 36 isconformal or close to conformal, wherein thickness T1 of the verticalportions of dose film 36 is close to thickness T2 of the horizontalportions of dose film 36. For example, the difference betweenthicknesses T1 and T2 may be smaller than 20 percent of both ofthicknesses T1 and T2. Dose film 36 has bottom portions at the bottomsof trenches 32, and sidewall portions on the sidewalls of semiconductorstrips 30. Dose film 36 also includes some portions on the sidewalls andtop surfaces of pad layer 22 and mask layer 24.

The dopant may be added by introducing the dopant-containing processgases in the precursors for forming dose film 36. For example, processgases such as BF₃, B₂H₆, Triethylborate (TEB), Trimethylphosphite(TMPi), PH₃, AsH₃, may be added, depending on what dopant is to be dopedin dose film 36. The concentration of the dopant in dose film 36 may bein the range between about 10¹⁴ atom/cm³ and about 10¹⁶ atom/cm³. Thedopant may be implanted with a rate in the range between about 1 kcps(kilo counts per second) and about 6 kcps. Thickness T1 may be in therange between about 5 percent and about 15 percent of spacing S1, whichis the spacing between neighboring semiconductor strips 30, so that thedopant diffused (in subsequent steps) from dose film 36 may have adesirable concentration in semiconductor strips 30.

In accordance with some embodiments of the present disclosure, a thermaltreatment 37A is performed to drive the dopant in dose film 36 intosemiconductor substrate 20 and semiconductor strips 30. In accordancewith alternative embodiments, thermal treatment 37A is not performed atthis time, while a thermal treatment is performed in a subsequent step.In accordance with some exemplary embodiments, thermal treatment 37A isused to form a well region for a FinFET, wherein the dopant in dose film36 is diffused into semiconductor substrate 20 and semiconductor strips30 to form well region 39. The conductivity type of the dopant may beopposite to the conductivity type of the subsequently formedsource/drain regions 54 (FIG. 8B). For example, when source/drainregions 54 are of p-type, and the respective FinFET is a p-type FinFET,the dopant in dose film 36 is of n-type, and vice versa. Thecorresponding diffused region (such as a well region) is schematicallyillustrated as the region 39, which includes semiconductor strips 30 andthe portion of semiconductor substrate 20 over dashed line 39′.

In accordance with some embodiments of the present disclosure, thermaltreatment 37A is performed using Rapid Thermal Anneal (RTA). Theannealing temperature may be in the range between about 800° C. andabout 1,200° C. The annealing duration may be in the range between about10 seconds and about 60 seconds. The anneal may be performed in achamber, wherein no oxygen-containing gas is introduced.

In accordance with alternative embodiments of the present disclosure,thermal treatment 37A is performed using a furnace thermal treatment.The annealing temperature may be in the range between about 400° C. andabout 700° C. The annealing duration may be in the range between about20 minutes and about 5 hours. The anneal may also be performed with noadditional oxygen-containing gas (other than the oxygen in the ambientgas (such as open air or clean air)) added.

Dielectric material 38 is then formed to fill the remaining portions oftrenches 32, resulting in the structure shown in FIG. 5. The respectivestep is shown as step 208 in the process flow shown in FIG. 10. Theformation method of dielectric material 38 may be selected from FlowableChemical Vapor Deposition (FCVD), spin-on coating, CVD, ALD,High-Density Plasma Chemical Vapor Deposition (HDPCVD), LPCVD, and thelike. In accordance with some embodiments, dielectric material 38 is notdoped with p-type and n-type dopants during its formation.

In accordance with some embodiments in which FCVD is used, a silicon-and nitrogen-containing precursor (for example, trisilylamine (TSA) ordisilylamine (DSA)) is used, and hence the resulting dielectric material38 is flowable (jelly-like). In accordance with alternative embodimentsof the present disclosure, the flowable dielectric material 38 is formedusing an alkylamino silane based precursor. During the deposition,plasma is turned on to activate the gaseous precursors for forming theflow able oxide.

After dielectric material 38 is formed, an anneal/curing is performed,which converts flowable dielectric material 38 into a solid dielectricmaterial. The solidified dielectric material 38 is also referred to asdielectric material 38.

In accordance with some exemplary embodiments of the present disclosure,the anneal is performed in an oxygen-containing environment. Theannealing temperature may be higher than about 200° C., for example, ina temperature range between about 200° C. and about 700° C. During thethermal treatment, an oxygen-containing process gas is conducted intothe process chamber in which wafer 100 is placed. The oxygen-containingprocess gas may include oxygen (O₂), ozone (O₃), or combinationsthereof. Steam (H₂O) may also be used, and may be used without oxygen(O₂) or ozone, or may be used in combination with oxygen (O₂) and/orozone. The treatment time may be between about 4 hours and about 7hours. As a result of the thermal treatment, dielectric layer 38 iscured and solidified.

Thermal treatment 37A, rather being performed in the step as shown inFIG. 4, may alternatively be performed after the anneal/curing ofdielectric material 38, which is shown as thermal treatment 37B in FIG.5. The thermal treatment 37A or 37B, in order to drive the dopant indose film 36 into semiconductor strip 30, may be performed at a highertemperature and for a shorter duration than the anneal for curingdielectric layer 38, so that the diffusing rate is increased, and thedamage caused by the thermal treatment is reduced. The treatment 37B maybe performed using a method selected from the same candidate methods andprocess conditions for thermal treatment 37A.

A planarization such as a Chemical Mechanical Polish (CMP) is thenperformed, as shown in FIG. 6. The respective step is shown as step 210in the process flow shown in FIG. 10. STI regions 40 are thus formed,which include the remaining portions of liner oxide 34, dose film 36,and dielectric region 38. Mask layer 24 may be used as the CMP stoplayer, and hence the top surface of mask layer 24 is substantially levelwith the top surface of dielectric regions 38 and the top edges of dosefilm 36.

Similarly, rather than performing the thermal treatments 37A and 37B,the thermal treatment may also be performed after the CMP, and thecorresponding thermal treatment is shown as 37C. The treatment 37C maybe performed using a method selected from the same candidate methods andprocess conditions for thermal treatment 37A.

In accordance with some embodiments of the present disclosure, a wellimplantation is performed to form well region 39. The implantation maybe tilted, as illustrated by arrows 41. In accordance with someembodiments, the well implantation is performed in addition to the welldiffusion as shown by 37A (FIG. 4), 37B (FIG. 5), or 37C (FIG. 6). Inaccordance with alternative embodiments, the well implantation 41 isperformed, and none of thermal treatment 37A, 37B, and 37C is performed.

The mask layer 24 as shown in FIG. 6 is then removed. Mask layer 24, ifformed of silicon nitride, may be removed in a wet process using hotH₃PO₄ as an etchant. Next, the structure shown in FIG. 6 is used to formsemiconductor fins through the recessing (etching back) of STI regions40, and pad layer 22 is also removed. The respective step is shown asstep 212 in the process flow shown in FIG. 10. The resulting structureis shown in FIG. 7. The recessing of STI regions 40 may be performedusing a dry etch process or a wet etch process. In accordance with someembodiments of the present disclosure, the recessing of STI regions 40is performed using a dry etch method, in which the process gasesincluding NH₃ and HF are used. In accordance with alternativeembodiments of the present disclosure, the recessing of STI regions 40is performed using a wet etch method, in which the etchant solution is adilution HF solution, which may have an HF concentration lower thanabout 1 percent.

The height H1 of the remaining STI regions 40 may be in the rangebetween about 10 percent and about 50 percent of height H2 ofsemiconductor strips 30. The protruding portions of semiconductor strips30, which protrude higher than the top surfaces of the remaining STIregions 40, become semiconductor fins 42.

In accordance with some embodiments of the present disclosure, after therecessing of STI regions 40, thermal treatment 44 is performed. Thermaltreatment 44 may be performed using a method selected from the samecandidate methods and process conditions for thermal treatment 37A,which include RTA and furnace anneal, for example.

As a result of thermal treatment 44, the dopant in dose film 36 isdiffused into the lower portion of semiconductor strips 30 to formanti-punch-through stoppers 46. The upper portion of semiconductorstrips 30 is not diffused with the dopant. The respective step is shownas step 214 in the process flow shown in FIG. 10. Anti-punch-throughstopper 46 is schematically shown as lower than dashed line 48 andhigher than dashed line 39′. During the thermal treatment, the dopantdiffused from the portions of dose film 36 on opposite sidewalls of thesame semiconductor strip 30 may be joined, so that the diffused dopantlaterally extends throughout semiconductor strips 30. Alternativelystated, the dopants may diffuse laterally beyond the middle verticalline of semiconductor strips 30. In accordance with some embodiments ofthe present disclosure, for forming the anti-punch-through stopper, theconductivity type of the dopant is the same as that of the well region,and is opposite to the conductivity of the subsequently formedsource/drain regions 54 (FIG. 8B). For example, when source/drainregions 54 are of p-type, and the respective FinFET is a p-type FinFET,the dopant in anti-punch-through stoppers 46 is of n-type, and viceversa.

In accordance with some embodiments, thermal treatment 44 is performed,while none of the thermal treatments 37A (FIG. 4), 37B (FIG. 5), and 37C(FIG. 6) is performed. In accordance with alternative embodiments, oneof thermal treatments 37A, 37B, and 37C is performed, while thermaltreatment 44 (FIG. 7) is not performed. In accordance with alternativeembodiments, one of thermal treatments 37A, 37B, and 37C is performed,and thermal treatment 44 (FIG. 7) is also performed. The dual thermaltreatment advantageously causes the formation of the well region 39 andanti-punch-through stopper 46 using the same dose film 36.

After STI regions 40 are recessed to form semiconductor fins 42, aplurality of process steps are formed on semiconductor fins 42, whichprocess steps may include well implantations, gate stack formation, aplurality of cleaning steps, and the like. FinFETs are thus formed. Therespective step is shown as step 216 in the process flow shown in FIG.10. An exemplary FinFET 52 is illustrated in FIG. 8A, wherein theillustrated gate stack 51 includes gate dielectric 49 on the topsurfaces and sidewalls of fins 42, and gate electrode 50 over gatedielectric 49. Gate dielectric 49 may be formed through a thermaloxidation, and hence may include thermal silicon oxide. The formation ofgate dielectric 49 may also include a deposition step, and the resultinggate dielectric 49 may include a high-k dielectric material or anon-high-k dielectric material. Gate electrode 50 is then formed on gatedielectric 49. The formation processes of these components are notdiscussed in detail. Gate dielectric 49 and gate electrode 50 may beformed using a gate-first approach or a gate-last approach. The detailsof the gate-first approach or the gate-last approach are not describedherein. The remaining components of FinFET 52, which include source anddrain regions and source and drain silicides (not in the illustratedplane), are then formed.

FIG. 8B illustrates a cross-sectional view of FinFET 52, wherein thecross-sectional view is obtained from the plane containing line 8B-8B inFIG. 8A. As shown in FIG. 8B, anti-punch-through stopper 46 and/or wellregion 39 are illustrated as extending into semiconductor fin 42 and thelower portion of semiconductor substrate 30, and into the portion ofsemiconductor substrate 20 directly underlying STI regions 40. Theconductivity type of the dopant in dose film 36 is opposite to theconductivity type of source/drain regions 54. Also, the conductivitytype of anti-punch-through stopper 46 and well region 39 is opposite tothe conductivity type of source/drain regions 54.

FIG. 11 illustrates a dopant profile, which schematically illustratesthe relative dopant concentration of the dopant. The X-axis representsthe distance from point 56A in the directions toward points 56B, asshown in FIG. 8A. As shown in FIG. 9, the dopant concentrations of thedopant are the highest in dose film 36 since the dopant is diffused awayfrom dose film 36. The dopant concentrations of the dopant continuouslydrop from dose film 36 to both point 56A, and from dose film 36 to point56B. It is appreciated that even if there is a well implantation, thedopant concentration resulted from the well implantation is lower thanthe dopant concentration caused by the diffusion since punch-throughstopper 46 desires higher dopant concentration than the well region.Accordingly, in the final device (for example, as shown n FIG. 8A), thetrend of the dopant concentration is not affected (although the valuesmay be affected) by the well implantation, and may still be representedby the curve shown in FIG. 11.

FIG. 9 illustrates a complementary MOS device formed in accordance withsome embodiments of the present disclosure. In accordance with someembodiments, n-type FinFET region 60A is used for forming n-type FinFET52A. STI region 40A is formed in FinFET region 60A, and includes dosefilm 36A. Dose film 36A is in-situ doped with a p-type dopant, and isformed on the respective liner oxide 34. P-type FinFET region 60B isused for forming p-type FinFET 52B. STI region 40B is in FinFET region60B. In accordance with some embodiments, STI region 40B includes dosefilm 36B and dose film 36A over dose film 36B. In accordance with otherembodiments, STI region 40B includes dose film 36B (but no dose film36A), with dielectric region 38 over and contacting dose film 36B. Dosefilm 36B is in-situ doped with an n-type dopant, and is formed on therespective liner oxide 34. Dose films 36A in FinFET regions 60A and 60Bare formed simultaneously. Dose films 36A and 36B as shown in FIG. 9 maybe formed by blanket forming dose film 36B in both n-type FinFET region60A and p-type FinFET region 60B, and removing dose film 36B from n-typeFinFET region 60A, followed by forming dose film 36A in both n-typeFinFET region 60A and p-type FinFET region 60B.

A thermal treatment is performed to diffuse the dopants in dose films36A and 36B as shown in FIG. 9 into semiconductor strips 30. The thermaltreatment may be performed after the recessing of STI regions as shownin FIG. 7. In n-type FinFET region 60A, the punch-through stopper 46A isformed as a p-type region. In p-type FinFET region 60B, thepunch-through stopper 46B is formed as an n-type region. The dopants inboth dose films 36A and 36B are diffused into semiconductor strips 30,which dopants have opposite conductivity types and hence neutralize eachother. Since dose film 36B is closer to semiconductor strips 30A, itsdopant dominates. In addition, the doping concentration of the n-typedopant in-situ doped into dose film 36B may be higher than the dopingconcentration of the p-type dopant in-situ doped into dose film 36A. Inthe resulting structure, punch-through stopper 46B is dominated by then-type dopant, and is shown as being of n-type.

In the resulting dopant concentration profile of STI region 40B, then-type dopant in-situ doped in dose film 36B still has the highestconcentration in dose film 36B after the thermal treatment, and thedopant concentrations increasingly decrease in regions away from dosefilm 36B. Similarly, the p-type dopant in-situ doped in dose film 36Astill has the highest concentration in dose film 36A after the thermaltreatment, and the dopant concentrations increasingly decrease inregions away from dose film 36A. The dopant concentration profile of STIregion 40A is shown in FIG. 11.

The embodiments of the present disclosure have some advantageousfeatures. By using the dose films to carry dopants, the bottom portionsof the semiconductor strips, which are difficult to dope usingimplantation, may be doped to have a desired high concentration.

In accordance with some embodiments of the present disclosure, a methodincludes etching a semiconductor substrate to form trenches, with aportion of the semiconductor substrate between the trenches being asemiconductor strip, and depositing a dielectric dose film on sidewallsof the semiconductor strip. The dielectric dose film is doped with adopant of n-type or p-type. The remaining portions of the trenches arefilled with a dielectric material. A planarization is performed on thedielectric material. Remaining portions of the dielectric dose film andthe dielectric material form STI regions. A thermal treatment isperformed to diffuse the dopant in the dielectric dose film into thesemiconductor strip.

In accordance with some embodiments of the present disclosure, a methodincludes etching a semiconductor substrate to form trenches, with aportion of the semiconductor substrate between the trenches being asemiconductor strip, forming a liner oxide on sidewalls of thesemiconductor strip, and depositing a dielectric dose film over theliner oxide. The dielectric dose film extends into the trenches, and thedielectric dose film is doped with a dopant of p-type or n-type. Theremaining portions of the trenches are filled with a dielectricmaterial. A planarization is performed on the dielectric material, andremaining portions of the liner oxide, the dielectric dose film, and thedielectric material form STI regions. The STI regions are etched back,and a top portion of the semiconductor strip protrudes over top surfacesof remaining portions of the STI regions to form a semiconductor fin. Athermal treatment is performed to diffuse the dopant in the dielectricdose film into a lower portion of the semiconductor fin.

In accordance with some embodiments of the present disclosure, a deviceincludes a semiconductor substrate and an STI region in thesemiconductor substrate. The semiconductor substrate includes asemiconductor strip, with a sidewall of the semiconductor stripcontacting a sidewall of the STI region. The STI region includes a lineroxide, which has a bottom portion over and contacting a top surface ofthe semiconductor substrate, and a sidewall portion contacting thesidewall of the semiconductor strip. The STI region further includes adielectric dose film over the liner oxide, and a dielectric region overthe dielectric dose film. A dopant of n-type or p-type is doped in thedielectric dose film, the liner oxide, and the semiconductor substrate,and dopant concentrations of the dopant decrease from the dielectricdose film to the liner oxide, and from the liner oxide to thesemiconductor strip.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: etching a semiconductorsubstrate to form trenches, with a portion of the semiconductorsubstrate between the trenches being a semiconductor strip; depositing ablanket dose film on a top surface and sidewalls of the semiconductorstrip, wherein the blanket dose film is formed of a dielectric materialdoped with a dopant of n-type or p-type; filling remaining portions ofthe trenches with a dielectric material, with the dielectric materialover the blanket dose film; performing a planarization on the dielectricmaterial, wherein remaining portions of the blanket dose film and thedielectric material form Shallow Trench Isolation (STI) regions; afterthe planarization, etching upper portions of the blanket dose film toform a dielectric dose film on a lower portion of a sidewall of thesemiconductor strip, wherein a top end of the dielectric dose film islower than a top surface of the semiconductor strip; and performing athermal treatment to diffuse the dopant in the dielectric dose film intothe semiconductor strip, wherein the thermal treatment results in thedopant to be diffused into a bottom portion of the semiconductor strip.2. The method of claim 1 further comprising performing an additionalthermal treatment to diffuse the dopant in the blanket dose film into atop portion of the semiconductor strip.
 3. The method of claim 1 furthercomprising: forming a gate stack on a top portion of the semiconductorstrip; and forming a source/drain region on a side of the gate stack. 4.The method of claim 3, wherein the dopant in the dielectric dose filmhas a conductivity type opposite to a conductivity type of thesource/drain region.
 5. The method of claim 1 further comprisingperforming a thermal oxidation on the semiconductor strip to form aliner oxide, wherein the dielectric dose film is formed over the lineroxide.
 6. The method of claim 5, wherein the dielectric material of theblanket dose film is different from a material of the liner oxide. 7.The method of claim 6, wherein the depositing the blanket dose filmcomprises depositing a silicon nitride film.
 8. The method of claim 1,wherein the depositing the blanket dose film comprises depositing asilicon oxide film.
 9. A method comprising: etching a semiconductorsubstrate to form trenches, with a portion of the semiconductorsubstrate between the trenches being a semiconductor strip; oxidizing asurface layer of the semiconductor strip to form a liner oxide onsidewalls of the semiconductor strip; depositing a dielectric dose filmover the liner oxide, wherein the dielectric dose film extends into thetrenches, and the dielectric dose film is doped with a first dopant of afirst conductivity type; etching the liner oxide and the dielectric dosefilm until top edges of the liner oxide and the dielectric dose film arelower than a top surface of the semiconductor strip; and performing athermal treatment to diffuse the first dopant in the dielectric dosefilm into the semiconductor strip.
 10. The method of claim 9 furthercomprising: filling remaining portions of the trenches with a dielectricmaterial to form Shallow Trench Isolation (STI) regions; and etchingback the STI regions, wherein the etching back the STI regions comprisesthe etching the liner oxide and the dielectric dose film, and whereinthe thermal treatment is performed to diffuse remaining portions of thedielectric dose film into a lower portion of the semiconductor strip.11. The method of claim 9, wherein a top portion of the semiconductorstrip protrudes over top surfaces of remaining portions of the lineroxide and the dielectric dose film to form a semiconductor fin, and thesemiconductor fin is substantially free from dopants diffused from thedielectric dose film.
 12. The method of claim 11 further comprising:forming a gate stack on the semiconductor fin; and forming asource/drain region on a side of the gate stack, wherein the forming thesource/drain region comprises doping a second dopant of a secondconductivity type opposite to the first conductivity type.
 13. Themethod of claim 9, wherein the first dopant is in-situ doped into thedielectric dose film when the dielectric dose film is deposited.
 14. Themethod of claim 9 further comprising: depositing an additionaldielectric dose film of a second conductivity type opposite to the firstconductivity type over the dielectric dose film, with the thermaltreatment further performed on the additional dielectric dose film. 15.A method comprising: etching a semiconductor substrate to form trenches,with a portion of the semiconductor substrate between the trenches beinga semiconductor strip; forming a liner oxide on sidewalls of asemiconductor strip; depositing a first dielectric dose film over theliner oxide, wherein the first dielectric dose film extends into thetrenches, and the first dielectric dose film is doped with a firstdopant of a first conductivity type; depositing a second dielectric dosefilm over the first dielectric dose film, wherein the second dielectricdose film is doped with a second dopant of a second conductivity typeopposite to the first conductivity type; and performing a thermaltreatment to diffuse the first dopant and the second dopant into thesemiconductor strip.
 16. The method of claim 15 further comprising:filling the trenches with a dielectric material, with the dielectricmaterial being over a bottom portion of the second dielectric dose film,wherein the first dielectric dose film, the second dielectric dose film,and the dielectric material in combination form an isolation region; andetching the isolation region, so that topmost edges of the firstdielectric dose film and the second dielectric dose film are lower thana top surface of the semiconductor strip.
 17. The method of claim 16,wherein the thermal treatment is performed after the isolation region isetched.
 18. The method of claim 15, wherein the thermal treatmentcomprises a rapid thermal anneal performed at a temperature betweenabout 800° C. and about 1,200° C., with the thermal treatment lastsbetween about 10 seconds and about 60 seconds.
 19. The method of claim15, wherein the thermal treatment comprises a furnace anneal performedat a temperature between about 400° C. and about 700° C., with thethermal treatment lasts between about 20 minutes and about 5 hours. 20.The method of claim 15, wherein the first dopant diffused from the firstdielectric dose film into the semiconductor strip has a higherconcentration in the semiconductor strip than the second dopant diffusedfrom the second dielectric dose film.